1. Field of the Invention.
When a motor vehicle is braked, kinetic energy is dissipated into the brake disks in the form of heat. In order to recover this kinetic energy, the current state of the art provides for regenerative braking systems. Devices of this type are able to recover the kinetic energy given off during braking and to convert this into electrical energy.
2. Description of Related Art.
In a motor vehicle, the regenerative braking device is connected to an electrical distribution circuit of the vehicle, which comprises a battery in which this electrical energy can be stored. This battery is usually a conventional lead/acid battery. The electrical distribution circuit returns the energy that is stored in the battery to the different electrical and electronic components in the motor vehicle.
In a general manner, the requirements that a motor vehicle has for electrical energy increase as the number of such items of electrical and electronic equipment increase. There are two possible ways of meeting the growing need of motor vehicles, firstly by increasing the power of the alternators and the storage capacity of the battery or secondly by improving the energy performance of the electrical supply system.
The regeneration of electrical energy by regenerative braking contributes to an increase in the mean output of the electrical supply system and increases the amount of energy available at the same nominal installed power.
Increasing the power of the alternators and the storage capacity of the battery involves a number of drawbacks in terms of costs, space, a problem of installation in a difficult location (under the bonnet) and also of weight.
Furthermore, the lead/acid batteries normally used in motor vehicles are not suitable for loading with very high current levels during a sufficient period to allow part of the energy given off by a braking device to be recovered. In this type of device, the management of the transitory energy given off by regenerative braking is not efficient enough to ensure that the voltage circulating in the electrical distribution circuit is properly regulated and the repeated charging of the lead/acid battery has an effect of prematurely ageing the latter.
Another system consists in equipping the electrical supply system of the vehicle with a second electrical distribution circuit having a different secondary storage system to that of the main battery or storage system. The second electrical distribution circuit sits alongside the first circuit with the principal storage system. The second circuit with the secondary storage system delivers a floating DC supply voltage and the first circuit with the main storage system delivers a low DC voltage supply, generally lower than that of the said floating voltage. In this way, a general two-layer electrical energy distribution is obtained.
The two storage systems are interconnected by means of a DC/DC reversible voltage converter. The function of the converter is to enable energy to be transferred between the two storage systems and the distribution circuits. An electrical current generator, comprising an alternator or a starter alternator coupled to the heat engine of the vehicle, directly supplies the both secondary storage system with electrical energy and, through the converter, the main storage system.
The practice of employing a pack of very high capacity condensers as a storage system is already known. These very high capacity condensers are usually known as “super-capacitors” or “super-condensers” to the expert in the sector. The secondary storage system, which is also referred to “the super-capacitor” in the following description, has the function of recovering as much electrical energy as possible when the electrical current generator operates in the form of regenerative braking.
In comparison with a conventional lead/acid battery, the super-capacitor can operate regardless of the number of charging/discharging cycles and the depth of these is not affected by the voltage level of the charge, which can vary significantly.
To select the range of regeneration voltage of the super-capacitor used in a motor vehicle, the limits of the starting voltage must be satisfied, as it is the starting phase that requires the greatest electrical output so as to guarantee a sufficiently high energy level for the starter-alternator operating in the starter mode to start the heat engine. When the driver starts the engine, the super-capacitor discharges and unless the braking system is operated to any great degree, the amount of electrical energy that is recovered is insufficient to fully charge the super-capacitor. If the vehicle stops and the driver attempts to restart it, the quality of the restart is lower with the super-capacitor than with the heat engine.
The solution known previously requires an interval so that energy can be regenerated in the super-capacitor at a level generally of between 18 and 24 V so that the associated upper limits associated with the starting of the heat engine can be satisfied.